Characterization of heterobimetallic and mixed-valence complexes of molybdenum(V) derived from bis(2-hydroxy-1-naphthaldehyde) malonoyldihydrazone

Article abstract of DOI:10.1134/S002247661205006X

Heterobimetallic complexes [UO₂Mo^V(CH ₂L)(hzd)(H₂O)₂] _n, [ZnMo ^V(CH₂L)(hzd)(H₂O)₂] _n and mixedvalence complexes [Mo^VIO₂Mo^V(CH ₂L)(hzd)(H₂O)₂] _n (where hzdH ₃ = inhH₃, n = 1; slhH₃, n = 2) are synthesized from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH₂LH ₄) and monometallic precursor complexes [Mo(CH₂LH ₂)(hzd)]·nH₂O (n = 0, 1) in ethanol. The composition of the complexes is established based on the data obtained from the elemental analysis. The structure of the complexes is discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR, and IR spectroscopic studies. All complexes have μB values in the range 1.59-1.64 B.M., slightly lower than that required for one unpaired electron. The heterobimetallic complexes show two bands, while mixed-valence complexes show only one band in the visible region assigned to the d-d transition. The g-values decrease in going from uranyl-to-molybdenyl-to-zinc complexes containing the isonicotinoyldiazenido (inh) group, however, no such regular trend is observed in the case of complexes containing the salicyloyldiazenido (slh) group in the coordination sphere. In all complexes, the principal dihydrazone ligand is present in the enol form as a bridging hexadentate ligand in the anti-cis configuration where hydrazide ligands are coordinated to the metal centre as a trinegative bidentate ligand in the diazenido form.

Full text of DOI:10.1134/S002247661205006X

Journal of Structural Chemistry. Vol. 53, No. 5, pp. 866-875, 2012
Original Russian Text Copyright © 2012 by A. Kumar, O. B. Chanu, R. Borthakur, R. A. Lal
CHARACTERIZATION OF HETEROBIMETALLIC
AND MIXED-VALENCE COMPLEXES OF MOLYBDENUM(V)
DERIVED FROM BIS(2-HYDROXY-1-NAPHTHALDEHYDE)
MALONOYLDIHYDRAZONE
1
A. Kumar, O. B. Chanu, R. Borthakur, and R. A. Lal
2
2
2
©
UDC 543.422:541.65:541.49:546.77
V
Mo (CH
V
Heterobimetallic complexes [UO
2
2
L)(hzd)(H
2 2 n 2 2 2 n
O) ] , [ZnMo (CH L)(hzd)(H O) ] and mixed-
VI V
valence complexes [Mo O Mo (CH L)(hzd)(H O) ] (where hzdH = inhH , n = 1; slhH , n = 2) are
2
2
2
2 n
3
3
3
synthesized from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH LH ) and monometallic
2 4
precursor complexes [Mo(CH
established based on the data obtained from the elemental analysis. The structure of the complexes is
discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR, and IR
spectroscopic studies. All complexes have μ values in the range 1.59-1.64 B.M., slightly lower than that
2 2 2
LH )(hzd)]⋅nH O (n = 0, 1) in ethanol. The composition of the complexes is
B
required for one unpaired electron. The heterobimetallic complexes show two bands, while mixed-valence
complexes show only one band in the visible region assigned to the d–d transition. The g-values decrease in
going from uranyl-to-molybdenyl-to-zinc complexes containing the isonicotinoyldiazenido (inh) group,
however, no such regular trend is observed in the case of complexes containing the salicyloyldiazenido
(slh) group in the coordination sphere. In all complexes, the principal dihydrazone ligand is present in the
enol form as a bridging hexadentate ligand in the anti–cis configuration where hydrazide ligands are
coordinated to the metal centre as a trinegative bidentate ligand in the diazenido form.
Keywords: synthesis, characterization, molybdenum(V), heterometal, hydrazone, spectroscopic studies.
INTRODUCTION
Molybdoenzymes are found in nearly almost all the organisms in the oxo-form and catalyze the redox-reactions
involving three oxidation states of molybdenum (Mo(IV), Mo(V), and Mo(VI)) to complete two-electron reduction-oxidation
processes [1]. In addition to the presence of molybdenum in oxidases, it also occurs in nitrogenase found in blue algae and
free living bacteria. In this enzyme, it is present in the non-oxo form together with iron and participates in the conversion of
atmospheric nitrogen into ammonia in the soil [2]. Thus, molybdenum is present either alone in enzymes in the mixed-
valence state or in combination with another metal giving the heterobimetallic character to the enzyme. Further, the Schiff
base heterobimetallic chemistry is an area of active research due to the importance of heterobimetallic complexes in catalysis
[3], and materials synthesis [4]. In catalysis, a heterobimetallic complex containing an electron deficient metal atom and an
electron rich metal atom presents the possibility of Lewis acid activation of a substrate molecule bound to the electron rich
1
Department of Chemistry, University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies;
2
Arvind.Kumar@sta.uwi.edu. Department of Chemistry, North Eastern Hill University, Shillong, Meghalaya, India;
ralal@rediffmail.com. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 53, No. 5, pp. 887-
895, September-October, 2012. Original article submitted November 5, 2011.
866 0022-4766/12/5305-0866

Fig. 1. Structure of the ligands: (a)
bis(2-hydroxy-1-naphthaldehyde)×
malonoyldihydrazone (CH LH ),
2
4
(b) isonicotinoylhydrazine (inhH3),
and (c) salicyloylhydrazine (slhH3).
metal centre [5, 6]. These complexes might be helpful in investigating the mutual influence of two metal centres on the
electronic, magnetic, and redox properties of such systems [7, 8].
In view of the above importance of the heterobimetallic and mixed-valence complexes of molybednum, the present
paper describes the synthesis of some mixed-valence and heterobimetallic complexes of molybdenum in the +5 oxidation
state, derived from polyfunctional CH LH , inhH , and slhH ligands (Fig. 1), in which zinc, molybdenyl, and uranyl ions are
2
4
3
3
the other metals. The complexes were mainly characterized based on the elemental analyses. The structure of the complexes
has been discussed in the light of molar conductivity, magnetic moment, electronic, EPR, and IR spectral data.
EXPERIMENTAL
Ammonium molybdate (NH
4 6 6 2 2 2 2 2 2
) Mo O24⋅4H O, UO (OAc) ⋅H O, Zn(OAc) ⋅H O, diethylmalonate, hydrazine hydrate,
2-hydroxy-1-naphthaldehyde, acetylacetone, and isonicotinoyl hydrazine (inhH ) were of Aldrich or equivalent grade.
3
Salicyloylhydrazine (slhH ) was prepared by reacting ethyl salicylate (1 mol) with hydrazine monohydrate (1 mol) and
3
recrystallized from hot benzene [9]. Malonoyldihydrazine (MDH) was prepared by reacting a dilute solution of
diethylmalonate in ethanol with hydrazine hydrate in the 1:2 molar ratio and recrystallized from hot water at ∼110°C.
2 2 2 2 2 2 2
MoO (acac) , [Mo(CH LH )(inh)]⋅H O, and [Mo(CH LH )(slh)] were prepared by literature methods [10, 11]. The
determination of uranium, molybdenum, and zinc was performed following the standard literature procedures [12]. Magnetic
susceptibility measurements were carried out on a Model 155 BAR vibrating sample magnetometer fitted with a Walker
Scientific L75BAL magnet. The molar conductance of the complexes was measured on a Direct Reading Conductivity meter-
–3
303 with a dip type conductivity cell at room temperature in DMSO (10 M). Molecular weights were determined in the DMSO
solution by the freezing point depression method. Infrared spectra were recorded on a paragon 500-model, Infrared
–1
spectrophotometer in the range 4000-350 cm in KBr discs. The electronic spectra of the complexes were recorded on a Milton
Roy Spectronic-21 spectrometer. The EPR spectra of the complexes in the powdered form at LNT were recorded at the X-band
frequency on a Varian E-112 E-line century series ESR spectrometer using TCNE (g = 2.00277) as the internal field marker.
Preparation of bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH LH ). The ligand bis(2-hydroxy-1-
2
4
naphthaldehyde)malonoyldihydrazone (CH LH ) was prepared by reacting a warm dilute solution of malonoyldihydrazide
2 4
(
1.32 g, 10.0 mmol) in ethanol (40 ml) with 2-hydroxy-1-naphthaldehyde (4.40 g, 25.0 mmol) also in ethanol (100 ml) at
0°C with gentle stirring for ca. 30-45 min. The yellow polycrystalline precipitate thus obtained was filtered and washed
repeatedly with warm ethanol and dried under vacuum. The observed C, H, N analysis values (calcd. values are in
7
867

parentheses) for C25
.5 g (79.5%).
Preparation of [MMo (CH L)(inh)(H O) ] {where M = UO (1), MoO (3), and Zn (5)} and [MMo (CH L)×
20 4 4
H N O (mol. wt., 440.45; m.p. 265°C) are C, 68.45 (68.17); H, 4.62 (4.58); N, 13.00 (12.72).Yield:
3
V
V
2
2
2
2
2
2
(slh)(H O) ] {where M = UO (2), MoO (4), and Zn (6)}. All the complexes have been synthesised under the dinitrogen
2 2 2 2 2
atmosphere.
In order to prepare complex [UO Mo (CH L)(inh)(H O) ] (1), the complex [Mo (CH LH )(inh)]⋅H O (0.684 g,
V
V
2
2
2
2
2
2
2
1.0 mmol) was taken in ethanol (50 ml) and stirred gently over a period of 10-15 min to make the suspension homogeneous.
This was added to a solution of UO (OAc) ⋅2H O (0.51 g, 1.20 mmol) in ethanol (50 ml) containing a trace of acetic acid,
2 2 2
maintaining the molar ratio at 1:1.2. The reaction mixture was further stirred for about 30 min, and refluxed for 3 h, which
produced a dark-red precipitate. The precipitate was filtered, washed with ethanol, ether and dried under vacuo over
anhydrous CaCl . Yield: 0.59 g (60%).
2
Complexes (3) and (5) were also prepared essentially by adopting the above procedure using MoO (acac) or
2 2
Zn(OAc)
2 2
⋅2H O instead of uranyl acetate. Yield 60-65%.
Similarly, complexes (2), (4), and (6) were also synthesised essentially by the above procedure using the precursor
V
complex [Mo (CH LH )(slh)] instead of the complex [Mo(CH LH )(inh)]⋅H O. Yield: 60-65%.
2
2
2
2
2
RESULTS AND DISCUSSION
The heterobimetallic complexes of Mo(V) were synthesised by adopting the strategies developed by Lintvedt et al.
V
13] and Davies et al. [14]. The monometallic molybdenum (V) complex [Mo (CH
V
[
2 2 2 2 2
LH )(inh)]⋅H O or [Mo (CH LH )(slh)]
was allowed to react with MoO (acac) or UO (OAc) ⋅2H O or Zn(OAc) ⋅2H O in the 1:1.2 molar ratio in absolute ethanol
2 2 2 2 2
2
2
under the inert atmosphere. All the complexes with their color, analytical, magnetic moment, molar conductance and
absorption data are given in Table 1. These complexes are air-stable and dark-red to orange in color and decompose or melt
with decomposition in the temperature range 260-280°C. The complexes are insoluble in water and common organic solvents
such as ethanol, methanol, ether, chloroform, dichloromethane, acetone, etc., but are soluble in CH CN, DMSO, and DMF.
3
Many attempts to crystallize the complexes either from the saturated solution or by diffusing ether into the saturated solution
in CH CN, DMSO, and DMF in a closed system resulted into amorphous solids. This prevented the X-ray analysis of these
3
–3
compounds. The molar conductance value of the complexes in the DMSO solution at 10 M concentration is in the range
–1
2
.5-7.0 Ohm cm mol suggesting that they are non-electrolytes in this solvent [15]. All complexes were heated at 110°C
–1
2
and 180°C and the weight loss was determined. None of the complexes showed weight loss at 110°C ruling out the possibility
of the presence of water molecules in their lattice structure. On the other hand, the complexes showed weight loss corresponding
to two water molecules at 180°C suggesting that they are coordinated to the metal centre. The vapours evolved at 180°C were
passed through a trap containing anhydrous CuSO that turned blue indicating that they originate from water molecules.
4
Molecular weight. The molecular weight data for the complexes (given in Table 1) have been determined in
spectral grade DMSO by the freezing point depression method. The experimental values of molecular weights for complexes
(1), (3), and (5) are very close to the values calculated based on their monomer formulas confirming their monomeric nature.
On the other hand, the experimental values of the molecular weights for the remaining complexes agree with their dimeric
character. However, the experimental values of the molecular weights are slightly higher than those calculated based on the
suggested dimer compositions. Such higher values of the molecular weights strongly suggest that the structure of the complexes
is altered in the strongly coordinating solvent as compared to that in the solid state. Most probably, the coordinated water
molecules are substituted by DMSO molecules, which accounts for the higher molecular weights of the complexes.
Magnetic moment. The magnetic moment values for heterobimetallic molybdenum(V) complexes (1), (2), (5), and
(6) and mixed-valence molybdenum(V)-molybdenum(VI) complexes (3) and (4) fall in the range 1.59-1.65 B.M. This value
lies well within the range reported for Mo(V) complexes without any significant metal-metal interaction. Slightly lower
868

TABLE 1. Complex, Color, Analytical, Molar Conductance, Magnetic Moment and Electronic Spectral
Data for Heterobimetallic and Mixed-Valence Molybdenum(V) Complexes
Elemental analyses: Found (calc), %
Electronic
spectral band
Molar cond. Magnetic
Sl.
Mol. Wt.
(calc)
–1
Λ
, ohm ⋅ moment
M
Complex (Color)
No.
Mo
M
C
H
N
λmax, nm (εmax,
2
cm ⋅mol
–1
μ
eff (BM)
3
–1
–1
dm ⋅mol ⋅cm )
CH
2
LH
4
—
—
—
68.45 4.62 13.00
68.18) (4.55) (12.73)
[UO Mo(CH L)(inh)× 1085± 70 10.15 25.05 38.95 2.41 9.88
(9.88) (24.49) (38.27) (2.47) (10.08)
—
—
320 (9500),
390 (15700)
340 (11225);
390 (12750),
(
1
4.5
1.65
2
2
(
H O) ] (dark-red)
2 2
(972)
4
30 (10250),
00 (7560),
50 (290)
350 (7980),
410 (10750),
00 (6720),
40 (310)
340 (8970),
420 (12750),
20 (5670),
30 (230)
420 (10750),
510 (6780),
40 (250)
360 (9750),
430 (12480),
40 (48)
340 (8950),
420 (10570)
30 (58)
5
6
2
[UO Mo(CH L)(slh)× 2150± 90 10.20 23.82 39.25 2.45
2
8.95
6.0
1.63
2
(H O) ] (Dark-red)
2 2 2
(9.73) (24.11) (38.91) (2.53) (8.51)
(1974)
5
6
3
[MoO Mo(CH L)(inh)× 930± 40 23.62
2
—
45.28 2.85 11.26
(44.82) (2.89) (11.81)
5.0
1.60
2
(H O) ] (red)
2 2
(23.13)
(830)
5
6
4
5
6
[MoO Mo(CH L)(slh)× 1880± 40 23.18
2
—
46.02 2.90 10.10
(45.44) (2.96) (9.94)
4.0
3.5
2.7
1.59
1.64
1.60
2
(H O) ] (dark-red)
2 2 2
(1690)
(22.72)
6
[ZnMo(CH L)(inh)× 890± 35 12.05 8.15 48.92 3.10 12.53
2
(H O) ] (orange)
2 2
(797.38)
(12.51) (8.36) (48.48) (3.13) (12.77)
6
[ZnMo(CH L)(slh)× 1750± 70 11.86 8.81 49.83 3.11 10.51
2
(
H O) ]
2
(1564.76)
(12.27) (8.36) (49.08) (3.20) (10.74)
2 2
(reddish-brown)
6
values than that required for one unpaired electron may be attributed to spin-orbit coupling.
Electronic spectra. The complexes have been characterized by electronic spectroscopy in DMSO solutions. The
electronic spectral bands together with molar extinction coefficients for the complexes have been set out in Table 1. The free
3
ligand CH LH shows two bands at 320 nm (εmax, 9500 dm ⋅mol ⋅cm ) and 390 nm (εmax, 15700 dm ⋅mol ⋅cm ) in the
–1
–1
3
–1
–1
2
4
region 300-400 nm. The band at 320 nm is assigned to the intraligand π→π* transition, while the band at 390 nm is assigned
to the n→π* transition, a characteristic of the naphthaldimine part of the ligand, as has been reported for several
monoacylhydrazones [16, 17]. The electronic spectra for the complexes also show two bands in the region 300-450 nm. Both
ligand bands at 320 nm and 390 nm showed a red shift on complexation to the metal. The absorption band appearing in the
region 340-390 nm in the absorption spectra of the complexes is attributed to the π→π* transition corresponding to the ligand
band at 320 nm. On the other hand, the band appearing in the region 410-430 nm may be assigned to arise due to the ligand
band at 390 nm and could be attributed to the n→π* transition. The red shift of the ligand bands by 20-70 nm and 20-40 nm
provides good evidence of the chelation of dihydrazone to the metal centre. The shift magnitude of the ligand bands on
complexation indicates strong bonding between the ligand and the metal centre. All of the complexes show strong bands in
the region 340-450 nm, which is indicative of their discrete molecularity [11, 18]. In addition to the ligand bands, complexes
(1) to (4) show two new bands in the regions 650-630 nm and 520-500 nm respectively. The bands in this region have been
869

TABLE 2. EPR Spectral Data for Heterobimetallic and Mixed-Valence Molybdenum(V) Complexes at
LNT in DMSO Solution
Sl.
A1 or A||
Complex
Temp
LNT
g
av
g
1
or g||
g
2
g3 or g⊥
A
av
A (g)
2
A3 or A⊥
–4 –1
(×10 cm )
No.
[
Mo(CH LH )(inh)]⋅H O
2 2 2
1.995 2.111
—
1.933
40
40
35
58
48
—
42
41
70
—
40
—
40
—
15
—
20
80
40
20
64
52
—
40
53
(
solid)
LNT
solid)
LNT
[
Mo(CH
2
LH )(slh)]
2
1.987 2.053 1.993 1.915
40
65
70
40
47
46
50
(
1
2
3
4
5
6
[UO
2
Mo(CH
dark-red)
Mo(CH L)(slh)(H
dark-red)
Mo(CH L)(inh)(H
red)
Mo(CH L)(slh)(H
dark-red)
[ZnMo(CH L)(inh)(H
orange)
[ZnMo(CH L)(slh)(H
reddish-brown)
2
L)(inh)(H
2
O)
2
]
2.096 2.171
—
1.968
(
(solid)
LNT
[UO
2
2
2
O)
2 2
]
1.975 2.111 1.980 1.833
(
(solid)
LNT
[MoO
2
2
2
O)
2
]
2.027 2.149
—
1.967
(
(solid)
LNT
[MoO
2
2
2
O)
2 2
]
2.008 2.106 1.980 1.938
(
(solid)
LNT
2
2
O)
2
]
1.995 2.005
—
1.965
(
(solid)
LNT
2
2
O)
2 2
]
2.030 2.125 2.031 1.933
(
(solid)
2
reported to correspond to ligand field transitions B → E and B → B respectively in D symmetry [19]. However, the
2
2
2
2
1
2
1
4h
3
–1
–1
molar absorptivity of the band in the region 520-500 nm lies in the range 7560-5670 dm ⋅mol ⋅cm . Such a high molar
extinction coefficient for this band suggests that it mainly arises due to charge-transfer transitions masking weak ligand-field
band occurring in this region. Further, the molar extinction coefficient for the band in the region 650-630 nm lies in the range
3
30-250 dm ⋅mol ⋅cm . The enhancement in the intensity of this band is probably due to the intensity borrowing from the
–1
–1
3
adjacent charge transfer band. On the other hand, heterobimetallic Zn–Mo(V) complexes (5) and (6) show a single band at
3
30 nm, and the molar extinction coefficient of this band is 48 dm ⋅mol ⋅cm and 58 dm ⋅mol ⋅cm respectively. In these
–1
–1
3
–1
–1
6
2
2
2 1
complexes, this band is ascribed mainly to the ligand field transition that is B → E in character. The position of the ligand
field bands remains almost the same as in the precursor monometallic complexes, indicating that the second metal centre
present in the complexes does not affect the energy gap of the first electronic excitation transition.
Electron paramagnetic resonance (EPR). All complexes have been characterized by EPR spectroscopy at LNT in
the solid state. Various EPR parameters for the complexes have been set out in Table 2. The EPR spectra for complexes (1),
(4), (5), and (6) have been shown in Fig. 2.
The two types of heterobimetallic and mixed-valence molybdenum complexes (1) to (6) show anisotropic spectra,
but still there are significant differences between the spectral features of these complexes which are dependent on the type of
the secondary ligands, i.e. isonicotinoylhydrazine and salicyloylhydrazine coordinated to the metal centres. The EPR spectra
of the complexes show the features similar to their corresponding precursor monometallic complexes [Mo(CH LH )(inh)]⋅
2
2
H O and [Mo(CH LH )(slh)][11]. This observation indicates that the essential features of the molybdenum(V) centre in these
2 2 2
complexes remain the same as those in their precursor monometallic complexes. The spectra of heterobimetallic
molybdenum(V) and mixed-valence Mo(V)–Mo(VI) complexes (1), (3), and (5), containing coordinated
isonicotinoyldiazenido groups in the coordination sphere, possess axial symmetry [20]. The g value for these complexes lies
|
|
in the range 2.171-2.055, while the g
mixed-valence Mo(V)–Mo(VI) complexes (2), (4), and (6), containing coordinated salicyloyldiazenido groups in the
coordination sphere, show complete anisotropy in their spectral features, and the g-values are in the regions 2.106-2.125,
⊥
value lies in the range 1.968-1.965. On the other hand, heterobimetallic Mo(V) and
870

Fig. 2. EPR spectra of the complexes at 293 K in the solid state: (a) 1;
b) 4; (c) 5; and (d) 6.
(
1.980-2.031, and 1.838-1.938 respectively. This feature suggests that the x, y, and z coordinate axes are highly misaligned,
which may be related to the coordination of the salicyloyl–OH group to the metal centre in complexes (2), (4), and (6).
Further, the EPR spectral features for these complexes are consistent with the rhombic symmetry of the complexes [21].
However, the derived g and A tensor components are different from those of the precursor complexes, which is obviously due
to the effect of another metal present in the same molecular unit of the complex. It is imperative to mention that the g and g
av
||
values decrease in going from UO –Mo(V) complex (1) to MoO –Mo(V) complex (3), and to Zn–Mo(V) complex (5).
2 2
Although, the g
observed in other complexes (2), (4), and (6) containing the coordinated salicyloyldiazenido group.
Infrared spectra. Structurally significant IR spectral bands for CH LH , precursor monometallic complexes, and
⊥
value for the complexes also decreases in the same order, but it is not so pronounced. No such trend is
2
4
heterobimetallic and mixed-valence Mo(V) complexes have been set out in Table 3. A comparison of the IR spectra of the
heterobimetallic complexes with those of the ligand and their precursor complexes suggests that dihydrazone is present in the
–1 –1
enol form in both types of the complexes. The ligand shows strong broad bands centred at 3450 cm , 3200 cm , and
–1
3047 cm . These bands are assigned to arise from the stretching vibrations of naphtholic-OH and secondary –NH groups
–1
respectively [11, 22]. Complexes (2), (4), and (6) show a strong broad band in the 3550-3000 cm region with peaks in the
–1 –1
420-3400 cm and 3220-3186 cm regions respectively, and the remaining complexes show only a single strong broad
3
–1 –1
band in the region 3550-3000 cm with a broad peak in the 3420-3400 cm region. The essential feature of the band in the
–1
region 3420-3400 cm in all complexes is similar in its shape and position to those commonly observed for water or ethanol
and accordingly, has been assigned to the ν(OH) mode. However, from its position and intensity it is not possible to
distinguish whether they are coordinated to the metal centre or present in the lattice. But the presence of a band in the region
–1
r
670-644 cm in the complexes assigned to the ρ mode of coordinated water suggests that these are water molecules that are
present in the first coordination sphere around the metal centre in the complexes rather than the ethanol molecules [23].
Moreover, all complexes show weight loss at 180°C corresponding to two water molecules lending credence to the suggested
number of coordinated water molecules. The fact that the weight loss occurs due to the water molecules was confirmed by
passing the ensuing vapors through a trap containing anhydrous copper sulphate that turned blue. This further ruled out the
possibility of the presence of ethanol molecules in the complexes and withal unambiguously establishes the water molecules
–1
coordinated to the metal centre. On the other hand, complexes (2), (4) and (6) show a new band in the region 3220-3212 cm ,
which is not observed in complexes (1), (3), and (5). This band is relatively broader than the common νNH band, which rules
871

TABLE 3. Infrared Spectral Data for Heterobimetallic and Mixed–Valence Molybdenum(V) Complexes
Sl.
2+
ν(MO )
2
Ligand/complex
ν(NCO)ν(N=N)
No.
CH LH
2
4
3450sbr 1699vs 1617vs 1532vs
200s 1661vs 1596vs
047s
3500- 1671 s 1618 s 1534 s
000sbr 1594 s
3395s 1669 m
212s
—
—
1278s 1030w
—
—
—
3
3
[
Mo(CH
LH )(inh)]H
2 2
2
O
—
—
—
1504 s 1272 s 1057 w
—
—
—
—
—
—
3
[
Mo(CH
LH )(slh)]
2 2
—
1535 s
1481 s
—
1031 w
3
1
2
3
4
[UO
2
Mo(CH
2
L)(inh)(H
2
O)
2
]
3550-
—
—
—
—
1661vs 1540s
1597vs
1060w 907vs 587m 473m
1508s 1272s 529w
(
dark-red)
3000sbr
[UO
2
Mo(CH
2
L)(slh)(H
2
2 2
O) ]
3550-
1617s 1539vs
1598s
—
1033w 919m 591m 459w
1486s 1292m 530m 433w
(
dark-red)
3300sbr
3
217s
] 3550-
3000sbr
[MoO
2
Mo(CH
2
L)(inh)(H
2
O)
2
1617vs 1544vs 1517m
1600vs
1039m 939vs 592w 464w
913vs 513w 434w
(red)
1486s
—
[MoO
2
Mo(CH
2
L)(slh)(H
2
O)
2 2
]
3400sbr
3213s
133s
1616s 1540s 1503m
1597vs
1066w 947s 595s 437w
1503w 1275m 906vs
(
dark-red)
3
5
6
[ZnMo(CH
orange)
[ZnMo(CH L)(slh)(H
reddish-brown)
2
L)(inh)(H
2
O)
2
]
3550-
3000sbr
3400sbr
3220s
—
—
1617vs 1540vs 1504s 1504s 1272m 1036w
1600vs
1616vs 1540s 1520s
1599vs
—
—
568w 442w
595w 470w
(
2 2
O) ]
1278m 1040w
2
2
(
1486m
3134s
out the possibility of its origin from secondary NH groups; instead it suggests that this band originates from the stretching
vibration of the OH group. The shape and position of this band is similar to that observed for complexes containing salicyloyl-
based hydrazides and hadrazones, where the salicyloyl–OH group has been reported to coordinate to the metal centre.
–1
–1
The ν(C=O) band present at 1699 cm and 1661 cm in free dihydrazone is absent in both types of heterobimetallic
and mixed-valence molybdenum complexes, indicating the collapse of the amide structure of dihydrazone as a result of
enolization on introduction of the second metal centre as a consequence of the heterometallic complex formation. The
ν(C=N) band appears as a couple of bands in these complexes, similar to those observed in free dihydrazone. This feature of
the C=N band observed in the complexes suggests that the two >C=N groups are inequivalent in these complexes. Such an
inequivalency in the >C=N groups arises due to the coordination of dihydrazone to the metal centre in the anti–cis
configuration, where the molecule is oriented in such a manner that half of hydrazine is out of the molecular plane and the
other half remains in the molecular plane [22]. Another interesting observation is that the ν(C=N) band shifts to a higher
–1
frequency by 1-3 cm in all complexes. This shift of the ν(C=N) band to higher frequencies indicates an increase in the >C=N
bond order. This increase in the bond order of the >C=N group in the complexes manifests the flow of electron density from the
naphthyl ring to the metal centre through the azomethine nitrogen atom. This process gives rise to a stronger azomethine
nitrogen-to-metal bond, which may be related to the nature of the metal ion bonded to the azomethine nitrogen atom.
872

–1
Complexes (3) to (6) show a band in the region 1520-1503 cm , which is characteristic of the newly created NCO
–
–1
group produced as a result of enolization of dihydrazone [23, 24]. A strong to very strong band in the 1544-1531 cm region
is observed for all the complexes, which is assigned to the ν(C–O)(naphtholate) vibration. The position of this band is
consistent with the bonding of the naphtholate oxygen atom to the metal centre. However, in complex (2), this band has a
–
contribution due to the stretching vibration of the NCO group produced as result of enolzation of dihydrazone. The β(C–O)
–1
–1
band appears in the region 1292-1272 cm in the complexes. The ν(N–N) band appears in the region 1095-1030 cm ,
similar to that in the precursor monometallic complexes indicating the coordination of only one hydrazinic nitrogen atom to
the metal centre [25].
–1
All complexes do not show any band in the region 1630-1700 cm , which could be assigned either to >C=O or
δNH
2
vibrations of the hydrazide ligand. Further, complexes (5) and (6) do not show any strong band in the region 850-
–1
2+
1000 cm due to the νMo=O vibrations of the cis-MoO2 group. The absence of any band characteristic of >C=O and –NH
2
–1
groups in the region 1630-1700 cm along with the νMo=O
t
vibration indicates the condensation of the –NH
2
group of
hydrazides with the Mo=O group. This is further confirmed by that these complexes show a new band in the region 1486-
–1
1
508 cm , which is a characteristic of the newly created (N=N) group [26, 27]. All these facts together account for the
presence of pyridoyldiazenido and salicyloyldiazenido groups in the coordination sphere of the complexes. It is assumed that
the parent metal ion, i.e., the Mo(V) ion retains the N O coordination chamber. This assumption regarding the retention of
2
2
V
the N O coordination chamber by Mo is entirely on IR spectroscopic evidences and is merely tentative.
2
2
–1
Mixed-valence MoO (VI)–Mo(V) complexes (3) and (4) show two new bands in the region 947-906 cm ; on the
2
–1
other hand, heterobimetallic UO (VI)–Mo(V) complexes (1) and (2) show only one band at 907 cm and 919 cm
2
–1
–1
respectively. The two bands observed in the region 947-907 cm for complexes (3) and (4) are assigned to the symmetric and
2
asymmetric vibrations of the cis-MoO2 group, while only one band is observed in this region at 907 cm and 919 cm for
+
–1
–1
2
+
complexes (1) and (2) is assigned to the ν
3
(UO2 ) vibration of the linear oxo-uranyl group respectively [28]. The absence of
any band assignable to the oxo-molybdenum group for complexes (1), (2), (5), and (6) suggests that molybdenum is present
–1
in the non-oxo form, similar to that in the precursor complexes. The IR spectra of the complexes in the region below 600 cm
are very complicated and have several other weak bands in addition to the ligand bands. This is obviously due to the split of
ligand vibrations as well as the presence of ν(M–O)(naphtholic), ν(M–O)(carbonyl), and ν(M–O)(aquo) bands. On the
–1
careful analysis of the spectra of the ligand and metal complexes in this region, the bands appearing in the 595-525 cm and
–1
474-433 cm regions are tentatively assigned to ν(M–O)(naphtholic) [29] and ν(M–O)(carbonyl) [30] respectively.
CONCLUSIONS
The heterobimetalic complexes of molybdenum(V) with dioxouranium(VI) and zinc(II) and mixed-valence
dioxomolybdenum(VI)-molybdenum(V) complexes were synthesised from dihydrazone and proton donor ligands.
Dihydrazone is coordinated to the metal centres in the anti–cis-configuration in all the complexes. In this form, the
naphthaldimine fraction of dihydrazone is bonded to one metal centre, while carbonyl oxygen atoms are bonded to the second
metal centre. The isonicotinoylhydrazide co-ligand is bonded to the molybdenum(V) centre in the condensed form as a
trinegative-bidentate ligand through the terminal azenido nitrogen atom and the carbonyl oxygen atom in the enol form, while
the salicyloylhydrazide co-ligand is bonded to the metal centre as a trinegative-tridentate ligand through the terminal azenido
nitrogen atom, enolic-carbonyl oxygen atom, and salicyloyl–OH group. The molybdenum(V) metal centre in the N O2
2
coordination chamber retains its original six-coordinate stereochemistry in heterobimetallic complexes (1), (3), and (5),
whereas it attains seven-coordinate pentagonal bipyramidal stereochemistry in complexes (2), (4), and (6).
The tentative structures of the complexes are shown in Fig. 3.
Arvind Kumar thanks to the Campus Research and Publication Fund, the University of West Indies, St. Augustine
and Ram A. Lal thanks UGC, New Delhi, India for the grant of financial assistance through the major research project.
873

2+
Fig. 3. Tentative structure of the complexes: (a) [MMo(CH L)(inh)(H O) ] (M = [O=U=O]
2 2 2
2
1), MoO2 (3), Zn (5)) and (b) [{MMo(CH L)(slh)(H O) } ] (M = [O=U=O] (2), MoO2
+
2+
2+
(
2 2 2 2
(4), Zn (6)).
REFERENCES
1
2
3
4
5
6
. G. Schwarz, R. R. Mendel, and M. W. Ribbe, Nature, 460, 839-847 (2009).
. B. K. Burgess and D. J. Lowe, Chem. Rev., 96, No. 7, 2983-3011 (1996).
. C. Liu, H. Zhang, W. Shi, and A. Lei, Chem. Rev., 111, No. 3, 1780-1824 (2011).
. J. H. Therston, A. Kumar, C. Hofmann, and K. H. Whitmire, Inorg. Chem., 43, No. 26, 8427-8436 (2004).
. G. M. Sammis, H. Danjo, and E. N. Jacobsen, J. Am. Chem. Soc., 126, No. 32, 9928/9929 (2004).
. S. Leelasubcharoen, P. A. Zhizhko, L. S. Kuzmina, A. V. Churakov, J. A. K. Howard, and G. I. Nikonov,
Organometallics, 28, No. 15, 4500-4506 (2009).
7. B. P. Greenwood, S. I. Forman, G. T. Rowe, C. H. Chen, B. M. Foxman, and C. M. Thomas, Inorg. Chem., 48, No. 13,
6
251-4560 (2009).
. M. Andruh, Chem. Commun., 47, No. 11, 3025-3042 (2011).
. H. Galoms, C. Cave, M. Miocque, P. Rinijard, G. Tran, and P. Binet, Eur. J. Med. Chem., 25, No. 9, 785-788 (1990).
8
9
1
0. O. A. Rajan and A. Chakravorty, Inorg. Chem., 20, No. 3, 660-664 (1981).
1. R. A. Lal, O. B. Chanu, R. Borthakur, M. Asthana, A. Kumar, and A. K. De, J. Coord. Chem., 64, No. 8, 1393-1410
2011).
1
(
1
1
1
1
1
2. A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, Longman, London (1973).
3. R. L. Lintvedt, W. E. Lynch, and J. K. Zehetmair, Inorg. Chem., 29, No. 16, 3009-3013 (1990).
4. J. B. Carlson, G. Davies, and P. Vouros, Inorg. Chem., 33, No. 11, 2334-2340 (1994).
5. W. J. Geary, Coord. Chem. Rev., 7, No. 1, 81-122 (1971).
6. A. A. R. Despaigne, J. G. da Silva, A. C. M. do Carmo, F. Sives, O. E. Piro, E. E. Castellano, and H. Beraldo,
Polyhedron, 28, No. 17, 3797-3803 (2009).
1
1
1
2
7. S. C. Chan, L. L. Koh, P. H. Leung, J. D. Ranford, and K. Y. Sim, Inorg. Chim. Acta, 236, Nos. 1/2, 101-108 (1995).
8. S. A. Patil and V. H. Kulkarni, Inorg. Chim. Acta, 95, No. 4, 195-199 (1984).
9. C. S. J. Chang, D. Callison, F. E. Mabbs, and J. H. Enemark, Inorg. Chem., 29, No. 12, 2261-2267 (1990).
0. S. B. Kumar and M. Chaudhury, Dalton Trans., No. 8, 2169-2174 (1991).
874

2
1. I. K. Dhawan and J. H. Enemark, Inorg. Chem., 35, No. 12, 4873-4882 (1996).
2. R. A. Lal, S. Choudhury, A. Ahmed, R. Borthakur, M. Asthana, and A. Kumar, Spectrochim. Acta, 75, No. 1, 212-224
2010).
3. M. P. Degaonkar, V. G. Puranik, S. S. Tavale, S. Gopinathan, and C. Gopinathan, Bull. Chem. Soc. Jpn., 67, No. 7,
797-1801 (1994).
4. R. A. Lal, M. Chakrabarty, S. Choudhury, A. Ahmed, R. Borthakur, and A. Kumar, J. Coord. Chem., 63, No. 1, 163-175
2010).
2
(
2
2
1
(
2
2
2
2
2
3
5. A. Braibanti, F. Dallavalle, M. A. Pellinghelli, and E. Leporati, Inorg. Chem., 7, No. 7, 1430-1433 (1968).
6. H. Kang, S. Liu, S. N. Saikh, T. Nicholson, and J. Zubieta, Inorg. Chem., 28, No. 5, 920-633 (1989).
7. M. D. Fitzroy, J. M. Frederiksen, K. S. Murray, and M. R. Snow, Inorg. Chem., 24, No. 20, 3265-3270 (1985).
8. J. Topich and J. O. Bachert III, Inorg. Chem., 31, No. 3, 511-515 (1992).
9. G. C. Percy, J. Inorg. Nucl. Chem., 37, No. 10, 2071-2073 (1975).
0. L. Sacconi, A. Sabatini, and P. Gans, Inorg. Chem., 3, No. 12, 1772-1774 (1964).
875